1. Field of the Invention
The present invention relates to a liquid-crystal display device of the active matrix system for reducing failure occurring when bonding substrates, and more particularly to a peripheral circuit integral type liquid-crystal display device.
2. Description of the Related Art
A conventional active matrix liquid-crystal display device is so designed as to control the optical characteristics such as a light transmission property of a liquid-crystal material which is held between a pair of pixel electrodes using the switching operation of a two-terminal element such as an MIN which is disposed in a pixel section in the form of a matrix or a three-terminal element such as a TFT, for display. In general, TFTs using amorphous silicon have been widely used for the switching element of the pixel electrodes.
However, because the mobility of the electric field effect of amorphous silicon is low to the degree of 0.1 to 1 cm Vs, the TFT using amorphous silicon cannot be disposed in a peripheral drive circuit that controls the TFT connected to the pixel electrode.
For that reason, in the conventional active matrix liquid-crystal device, the peripheral drive circuit which is made up of a semiconductor integrated circuit is attached externally to a liquid-crystal panel through the tape automatic bonding (TAB) technique or the chip on glass (COG) technique.
FIG. 16 is a front view showing the outline of an active matrix liquid-crystal panel in accordance with a first conventional example, to which a peripheral drive circuit is attached externally. As shown in FIG. 16, scanning lines 2 and signal lines 3 are disposed on an element substrate 1 made of, for example, glass or quartz in a matrix, and in a pixel section 4, pixel electrodes and a switching pixel TFT for the pixel electrodes are connected to each of the cross portions of those wirings. The scanning lines 2 and the signal lines 3 extend up to the outside of a sealing material region 5, respectively, and for that reason, the number of wirings which are transverse to the sealing material is as much as the number of the scanning lines 2 and the signal lines 3 at the minimum. The ends of those wirings form extension terminals 6 as they are, and the extension terminals 6 are connected with a peripheral drive circuit not shown. Furthermore, the element substrate 1 is joined to an opposite substrate not shown through the sealing material disposed in the sealing material region 5, and a liquid-crystal material is interposed between those substrates through the sealing material.
Also, in recent years, in order to obtain a TFT with a large mobility of the electric field effect, a technique for fabricating the TFT using crystalline silicon has been intensively researched. The TFT using the crystalline silicon enables operation which is remarkably higher than that of an amorphous silicon TFT, and not only a TFT of NMOS but also a TFT of PMOS are obtained from crystalline silicon in the same manner, thereby being capable of obtaining a CMOS circuit. Hence, a display section as well as the peripheral drive circuit can be fabricated on the same substrate.
FIG. 17 is a front view showing the outline of an active matrix liquid-crystal display device in accordance with a second conventional example, in which a peripheral drive circuit and a display section are integrated on a panel. As shown in FIG. 17, a pixel section 12 is disposed on an element substrate 11 made of, for example, glass or quartz, and a signal line drive circuit 13 is disposed on an upper side of the pixel section 12 around the pixel section 12, and a scanning line drive circuit 14 is disposed on a left side thereof. Signal lines 15 and scanning lines 16 are connected to the signal line drive circuit 13 and the scanning line drive circuit 14, respectively. The signal lines 15 and the scanning tines 16 form a lattice in the pixel section 12, and the ends of the signal lines 15 and the scanning lines 16 extend up to the outside of the sealing material region 17 and are connected with a control circuit, a power supply not shown, or the like. Also, the element substrate 11 and the opposite substrate 18 are joined to each other through the sealing material formed in the sealing material region 17, and a liquid-crystal material is interposed between those substrates 11 and 14 by the shape of the sealing material. Further, an external terminal 19 is disposed on the element substrate 11.
In the first conventional example shown in FIG. 16, the wiring structure around the pixel section 4 is symmetrical with respect to top and down as well as right and left on the paper surface with the result that the steps of the sealing section are made uniform, thereby being capable of making an interval between the substrates uniform.
However, in the first conventional example, because the peripheral drive circuit is connected to the outside of the sealing material, there are a lot of wirings that are transverse to the sealing material, and moisture enters from the interfaces between the wirings which connect the drive circuit to the pixel section and the sealing material, resulting in such a problem that the liquid-crystal surface material is deteriorated. Also, because the peripheral drive circuit is disposed outside, the device is made large in size.
In order to eliminate those problems, the peripheral drive circuit integral type active matrix liquid-crystal display device in accordance with the second conventional example shown in FIG. 17 has a peripheral drive circuit disposed inside the sealing material region 17. Also, a one-side drive system is generally adopted without any provision of a redundant circuit. For that reason, as shown in FIG. 17, since wirings are transversal to the sealing material only on the right side and the lower side of the element substrate 11, the wiring structure has no symmetry with respect to top and down as well as right and left on the paper surface, the step of the sealing material on the peripheral drive circuit side is different from that of the sealing material on a wiring extending side. Hence, in bonding the substrates together, because no pressure is uniformly applied to the substrate, it is difficult to make an interval between the substrates uniform. As a result, nonuniformity occurs on display, or an image quality is deteriorated.
In particular, because the step of the sealing material on the peripheral drive circuit side is low, when bonding the substrates together, there may be a case in which the wirings are short-circuited between the top and the bottom in the peripheral drive circuit, thereby being liable to generate a line defect. Those problems lead to additional causes such as the deterioration of the yield of the peripheral drive circuit integral type liquid-crystal display device, or the lowering of the reliability.
Also, in the pixel element, a most projected portion is in a region where the scanning lines and the signal lines are superimposed one on another, and in the region, not only the scanning line, the signal line, an inter-layer insulation film for separating those lines from each other, but also a pixel electrode, a black matrix and so on are laminated one on another. In general, columnar fibers for maintaining the interval between the substrates are mixed with the sealing material. The dimensions of the fiber are set to values obtained by taking into consideration the margin in addition to the thickness of the projected portion in the pixel section and the dimensions of spacers dispersed inside the sealing material in such a manner that the step of the sealing material is higher in level than the pixel section. However, if the spacer is disposed on the projected portion of the pixel section, the pixel portion becomes higher than the sealing material, and when the substrates are bonded together under this state, the scanning lines and the signal lines are short-circuited between the top and the bottom through the spacers, thereby causing the point defect and the line defect.